1. Field of the Invention
Aspects of the present invention relate to a servo controlling apparatus and method of a holographic information recording/reproducing system. More particularly, aspects of the present invention relate to a servo controlling apparatus and method of a holographic information recording/reproducing system that provide a more stable control between movements during a recording operation on a plurality of layers.
2. Description of the Related Art
Recently, information storing technology that uses a hologram has been in the limelight. An information storing method using a hologram stores information in the form of an optical interference pattern in an inorganic crystal or a photo polymer material sensitive to light. The optical interference pattern is formed using two laser beams having an interference characteristic. That is, an interference pattern formed when a reference light and a signal light having different paths, respectively, interfere with each other generates a chemical or physical change on a photosensitive storage medium to record information. To reproduce the information from the interference pattern recorded in this way, reference light similar to the light that was used when the information was recorded is illuminated onto the interference pattern recorded in the storage medium. When the reference light is illuminated onto the interference pattern, diffraction caused by the interference pattern is generated such that the signal light is recovered and the recorded information is reproduced. The hologram information storing technology includes a volume holography method in which information is recorded and reproduced on a page unit basis using volume holography, and a micro holography method in which information is recorded and reproduced on a single bit basis using micro holography. In the volume holography method, large scale information is processed simultaneously, but in this case, it is desirable to control the optical system very precisely. Therefore, the volume holography method has been too difficult to be commercialized as an information storing apparatus for general consumers.
On the other hand, in the micro holography method, two condensed beams interfere with each other at a focal point to form a fine interference pattern. The focal lengths of the two condensed beams are moved on the plane of a storage medium to form a plurality of interference patterns such that information is recorded on one information layer. Further, in the micro holography method, a plurality of information layers are formed in the depth direction of the storage medium such that information is recorded three-dimensionally in a storage medium. To record information over multiple layers, a signal light optical system moves a focus at which the signal light is condensed in the depth direction, and simultaneously, a reference light optical system moves a focus at which reference light is condensed to the same position. However, according to a conventional micro holographic recording/reproducing system, disturbance is generated in the optical pickup control in radial and tangential directions when the foci of the reference light and the signal light change between information layers on which information is recorded, so that the foci of the reference light and the signal light deviate from each other.